1. Field of the Invention
The present invention relates to a multi-axis magnetic lens and variants thereof used for focusing a plurality of charged particle beams individually and in parallel. More particularly, it relates to a multi-axis magnetic lens acting as an objective lens or a condenser lens, or a transfer lens in a multi-beam apparatus which uses a plurality of electron beams to in parallel expose patterns onto or inspect defects on a wafer or a mask in semiconductor manufacturing industry.
2. Description of the Prior Art
In semiconductor manufacturing industry, an electron beam has been used to expose patterns onto or inspect defects on a wafer or a mask since critical feature dimensions of patterns or defects have been beyond the competent ability of a photon beam. The reason is that an electron beam, due to its short wavelength, can offer superior spatial resolution compared to a photon beam. However, such a superior spatial resolution will be fundamentally deteriorated by electron interaction or called as Coulomb Effect as the electron beam current is increased to obtain a high throughput competent for mass production.
For mitigating the limitation on throughput, a promising solution is to use a plurality of electron beams each with a small current instead of using one electron beam with a large current. For this solution, the plurality of electron beams can be respectively focused by a plurality of single-axis magnetic/electrostatic lenses in a conventional manner or a plurality of magnetic sub-lenses of a multi-axis magnetic lens as Maekawa et al first proposed in the U.S. Pat. No. 3,715,580 as early as in 1971. Compared with the first way, the later way can even halve the interval between every two adjacent beams, thereby almost doubling the throughput.
FIGS. 1A and 1B respectively illustrate the configuration and the magnetic field distribution of the multi-axis magnetic lens 100 proposed in U.S. Pat. No. 3,715,580. The multi-axis magnetic lens 100 comprises one common exciting coil 44, one yoke 43, and two parallel magnetic conductor plates 41 and 42 with a plurality of through round holes in pairs. When an electric current is exerted into the coil 44, between a pair of coaxial through round holes, a magnetic axisymmetric field (round-lens field) will be generated along the coincident central axes thereof (such as 31 in FIG. 1B), and consequently one magnetic sub-lens such as 30 is formed therebetween. The magnetic sub-lens module with respect to the magnetic sub-lens is therefore formed by the pair of coaxial through round holes, and called as the first-type magnetic sub-lens module hereafter for the sake of clarity. The optical axis of the magnetic sub-lens coincides with the central axis of the magnetic sub-lens module. The multi-axis magnetic lens 100 therefore comprises a plurality of first-type magnetic sub-lens modules and consequently can form a plurality of magnetic sub-lenses therein, such as sub-lenses 10, 20 and 30. The two magnetic conductor plates 41 and 42 function as pole pieces of the magnetic sub-lens modules. The magnetic sub-lenses 10, 20 and 30 respectively focus the charged particle beams 1, 2 and 3 propagating along the optical axes thereof.
In the foregoing multi-axis magnetic lens 100, the magnetic flux leakage between each pair of coaxial through round holes depends on the positions thereof on the magnetic conductor plates 41 and 42, geometrical shapes and magnetic permeability of the magnetic conductor plates 41 and 42, and the distribution of all the through round holes on the magnetic conductor plates 41 and 42. Hence, each magnetic sub-lens fundamentally generates not only a pure round-lens field but also a lot of non-axisymmetric transverse field components or called as high order harmonics, such as dipole field and quadrupole field. Only the round-lens field is necessary for focusing an electron beam, and the other components are undesired due to generating additional aberrations. To compensate the influence of each high order harmonic, at least one additional element generating the same type field is required and obviously the additional element will increase the volume and the complication of the multi-axis magnetic lens. In addition, even if all the through round holes are same in geometry, all the magnetic sub-lenses are different in the round-lens fields because of the differences in magnetic flux flowing through each magnetic sub-lens module. The closer the magnetic sub-lens module is to the geometrical center of the magnetic conductor plates 41 and 42, the weaker the round-lens field the magnetic sub-lens module produces. For example, the magnetic sub-lens 20 has a weaker round-lens field than the magnetic sub-lens 10 if the through round holes of both are equal in dimensions. The differences in round-lens fields of magnetic sub-lenses are not desired due to incurring beam defocusing for a specific imaging plane with respect to the plurality of electron beams, such as the same beams 1, 2 and 3 respectively passing through the sub-lens 10, 20 and 30 will not be focused onto a same plane.
Many scientists propose methods to fundamentally mitigate or even eliminate the two issues per se. Lo et al. in U.S. Pat. No. 6,750,455 uses a plurality of dummy holes to improve the local structure symmetry of each sub-lens. However this method makes the multi-axis magnetic lens system bulky. Chen et al. propose one method in U.S. Pat. Nos. 8,003,953, 8,294,095 and 8,445,862 and the cross reference, which can fundamentally mitigate or even eliminate the high order harmonics per se of each magnetic sub-lens and the differences among round-lens fields of all the magnetic sub-lenses.
The method uses three steps to mitigate or even eliminate the high order harmonics per se of each magnetic sub-lens, as shown by the multi-axis magnetic lens 200 in FIG. 2A. The first step, as the most profound step, is to insert a magnetic round ring (such as 12) inside one through hole of each first-type magnetic sub-lens module (such as 10) with a radial gap (such as 14). The radial gap can be vacuum or filled of non-magnetic or weakly-magnetic conductor material, which keeps a strong magnetic coupling between the magnetic round ring and the corresponding magnetic conductor plate and effectively weakens the non-axisymmetric distribution of magnetic scalar potential inside the magnetic round ring. Consequently, a magnetic field will be leaked out through the magnetic-circuit gap between the two magnetic round rings and distribute along the optical axis of the magnetic sub-lens. Out of the magnetic field, the axisymmetric component (round-lens field) is strong enough and the non-axisymmetric transverse field components are almost eliminated. With this way, the through holes are not necessary round in shape. The magnetic sub-lens module with respect to the magnetic sub-lens therefore is formed by the pair of through holes, the magnetic round rings therein and the radial gaps therebetween, and is called as the second-type magnetic sub-lens module hereafter for the sake of clarity. The magnetic round rings functionally are magnetic pole pieces and can flexibly shape the magnetic circuit gap for a specific application, such as an axial magnetic-circuit gap of a magnetic condenser sub-lens or a radial magnetic-circuit gap of a magnetic immersion objective sub-lens.
The second step is to extend one of the two magnetic round rings of each magnetic sub-lens module into the other so as to further eliminate the high order harmonics therebetween. If the extended magnetic round ring fully goes through the through hole in which the other magnetic round ring is inserted, in some cases the other magnetic round ring can even be removed for the sake of simplification in manufacturing. In the magnetic sub-lens 10 of the multi-axis magnetic lens 300 shown in FIG. 2B, the upper magnetic round ring 12 fully crosses over the upper and lower holes and the lower magnetic round ring 13 in FIG. 2A therefore can be removed. The third step, as one global magnetic-shielding means, is to place two magnetic-shielding plates 51 and 52 respectively above and below the two parallel magnetic conductor plates 41 and 42 so as to reduce the high order harmonics distributing in the areas above and below the plurality of second-type magnetic sub-lens modules. Furthermore, the differences of the radial gaps of all the magnetic sub-lens modules can be specifically designed to mitigate or even eliminate the round-lens field differences among all the magnetic sub-lenses.
Therefore, for focusing a charged particle beam, each magnetic sub-lens of the multi-axis magnetic lenses proposed by Chen et al. can provide performance as good as a conventional single-axis magnetic lens. As well known, the performance is desired to keep stable both in a short term and in a long term. The magnetic field of a magnetic sub-lens depends on the common coil excitation (product of coil turns T and coil current I) and the structure of the magnetic sub-lens module. The coil in a multi-axis magnetic lens is likely longer than the coil in a conventional single-axis magnetic lens, thereby generating more heat to deform the structure of the magnetic sub-lens module. The variation of the coil current varies the common coil excitation. These factors will make the magnetic field of each magnetic sub-lens unstable. Accordingly, a new multi-axis magnetic lens, which can provide magnetic sub-lens fields with high stabilization, is needed.